This invention relates to lead-acid storage batteries, and specifically provides for a novel apparatus by which the condition of the battery cells can be readily ascertained from outside the cell container to facilitate cell maintenance.
A lead-acid storage battery cell is comprised of a plurality of plates (anodes and cathodes) immersed in an acid electrolyte. The plates are constructed of a suitable lead alloy, such as lead-calcium. The particular lead alloy employed depends upon the specific battery application, since different alloys offer different battery characteristics: for example, cycling time and discharge rate. The plates are constructed as a grid so as to optimize the active surface area, and the grids are filled with the active battery material in a paste form.
There are numerous factors contributing to the gradual diminution in usable battery life. The most notable of these being anode grid enlargement. As noted in the article titled, Positive Grid Design Principles, The Bell System Technical Journal, 1279 (1970), by minimizing the anode enlargement the usable battery life can be extended. This is particularly true of those batteries employed under float charge conditions, for example, as emergency standby power sources in the telephone industry. As noted in that article, grid enlargement results from the slow growth of a lead-oxide deposit within the anode grid structure, which having a specific volume that is 21% greater than that of the lead alloy, thus requires greater space in the grid structure than the lead consumed in producing the oxide. The consequential grid enlargement can produce among other things plate shorting and post and jar cracking. More importantly, this can result in cracks in the grid which thereby subject the balance of the grid to higher than normal electrical currents. The higher electrical currents can generate local hot spots in the grid which can produce further grid fragmentation or cracking. This fragmentation process is thus best defined as being regenerative once it starts and is a particular problem in those cells that are float charged by maintaining a constant voltage across the battery cells of 2.17 volts, this being 110 mv. above the open circuit cell voltage of 2.060 volts.
Accordingly, the slow enlargement of the cell anodes represents the slow but continuous decrease in cell life, even if the cell is not in use.
Thus it is an object of the present invention to utilize the relative change in anode size over time to produce an indication of remaining cell life which is both quick and reliable. This is particularly beneficial in standby power situations under which conditions the cells must be maintained in an optimum condition at all times without the benefit of operational testing. By utilizing the indicator of the present invention, those batteries having substantially enlarged anodes are easily determined and can be routinely removed from service and replaced.